Xylofuranosly-containing nucleoside phosphoramidites and polynucleotides

ABSTRACT

Novel xylo nucleoside or xylo nucleotide analogs, polynucleotides comprising xylo nucleotide substitution, processes for their synthesis and incorporation into polynucleotides.

This is a continuation of application Ser. No. 09/960,192 filed on Sep.21, 2001 now U.S. Pat. No. 6,489,465, which is a continuation of Ser.No. 09/135,964 filed on Aug. 18, 1998 now U.S. Pat. No. 6,316,612, whichclaims the benefit of Ser. No. 60/056,808 filed on Aug. 22, 1997.

BACKGROUND OF THE INVENTION

This invention relates to novel nucleoside or nucleotide analogs, andprocesses for their synthesis and incorporation into polynucleotides.

The following is a brief description of nucleoside analogs. This summaryis not meant to be complete but is provided only for an understanding ofthe invention that follows. This summary is not an admission that all ofthe work described below is prior art to the claimed invention.

Nucleoside modifications of bases and sugars, have been discovered in avariety of naturally occurring RNA (e.g., tRNA, mRNA, rRNA; reviewed byHall, 1971 The Modified Nucleosides in Nucleic Acids, ColumbiaUniversity Press, New York; Limbach et al., 1994 Nucleic Acids Res. 22,2183). In an attempt to understand the biological significance,structural and thermodynamic properties, and nuclease resistance ofthese nucleoside modifications in nucleic acids, several investigatorshave chemically synthesized nucleosides, nucleotides andphosphoramidites with various base and sugar modifications andincorporated them into oligonucleotides.

Uhlmann and Peyman, 1990, Chem. Reviews 90, 543, review the use ofcertain nucleoside modifications to stabilize antisenseoligonucleotides.

Usman et al., International PCT Publication Nos. WO/93/15187; and WO95/13378; describe the use of sugar, base and backbone modifications toenhance the nuclease stability of enzymatic nucleic acid molecules.

Eckstein et al., International PCT Publication No. WO 92/07065 describethe use of sugar, base and backbone modifications to enhance thenuclease stability of enzymatic nucleic acid molecules.

Grasby et al., 1994, Proc. Indian Acad. Sci., 106, 1003, review the“applications of synthetic oligoribonucleotide analogues in studies ofRNA structure and function”.

Eaton and Pieken, 1995, Annu. Rev. Biochem., 64, 837, review sugar, baseand backbone modifications that enhance the nuclease stability of RNAmolecules.

Rosemeyer et al., 1991, Helvetica Chem. Acta, 74, 748, describe thesynthesis of 1-(2′-deoxy-β-D-xylofuranosyl) thymine-containingoligodeoxynucleotides.

Seela et al., 1994, Helvetica Chem. Acta, 77, 883, describe thesynthesis of 1-(2′-deoxy-β-D-xylofuranosyl) cytosine-containingoligodeoxynucleotides.

Seela et al, 1996, Helvetica Chem. Acta, 79, 1451, describe thesynthesis xylose-DNA containing the four natural bases.

The references cited above are distinct from the presently claimedinvention since they do not disclose and/or contemplate the synthesis ofxylofuranosyl nucleoside phosphoamidites and polynucleotides comprisingsuch nucleotide modifications of the instant invention.

SUMMARY OF THE INVENTION

This invention relates to a compound having the Formula I:

wherein, R₁ is OH, O—R₃, where R₃ is independently a moiety selectedfrom a group consisting of alkyl, alkenyl, alkynyl, aryl, alkylaryl,carbocyclic aryl, heterocyclic aryl, amide and ester; C—R₃, where R₃ isindependently a moiety selected from a group consisting of alkyl,alkenyl, alkynyl, aryl, alkylaryl, carbocyclic aryl, heterocyclic aryl,amide and ester; halo, NHR₄ (R₄=alkyl (C1-22), acyl (C1-22), substitutedor unsubstituted aryl), or OCH₂SCH₃ (methylthiomethyl), ONHR₅ where R₅is independently H, aminoacyl group, peptidyl group, biotinyl group,cholesteryl group, lipoic acid residue, retinoic acid residue, folicacid residue, ascorbic acid residue, nicotinic acid residue,6-aminopenicillanic acid residue, 7-aminocephalosporanic acid residue,alkyl, alkenyl, alkynyl, aryl, alkylaryl, carbocyclic aryl, heterocyclicaryl, amide or ester, ON=R₆, where R₆ is independently pyridoxalresidue, pyridoxal-5-phosphate residue, 13-cis-retinal residue,9-cis-retinal residue, alkyl, alkenyl, alkynyl, alkylaryl, carbocyclicalkylaryl, or heterocyclic alkylaryl; B is independently a nucleotidebase or its analog or hydrogen; X is independently aphosphorus-containing group; and R₂ is independently blocking group or aphosphorus-containing group.

Specifically, an “alkyl” group refers to a saturated aliphatichydrocarbon, including straight-chain, branched-chain, and cyclic alkylgroups. Preferably, the alkyl group has 1 to 12 carbons. More preferablyit is a lower alkyl of from 1 to 7 carbons, more preferably 1 to 4carbons. The alkyl group may be substituted or unsubstituted. Whensubstituted the substituted group(s) is preferably, hydroxy, cyano,alkoxy, NO₂ or N(CH₃)₂, amino, or SH.

The term “alkenyl” group refers to unsaturated hydrocarbon groupscontaining at least one carbon—carbon double bond, includingstraight-chain, branched-chain, and cyclic groups. Preferably, thealkenyl group has 1 to 12 carbons. More preferably it is a lower alkenylof from 1 to 7 carbons, more preferably 1 to 4 carbons. The alkenylgroup may be substituted or unsubstituted. When substituted thesubstituted group(s) is preferably, hydroxyl, cyano, alkoxy, NO₂,halogen, N(CH₃)₂, amino, or SH.

The term “alkynyl” refers to an unsaturated hydrocarbon group containingat least one carbon—carbon triple bond, including straight-chain,branched-chain, and cyclic groups. Preferably, the alkynyl group has 1to 12 carbons. More preferably it is a lower alkynyl of from 1 to 7carbons, more preferably 1 to 4 carbons. The alkynyl group may besubstituted or unsubstituted. When substituted the substituted group(s)is preferably, hydroxyl, cyano, alkoxy, ═O, ═S, NO₂ or N(CH₃)₂, amino orSH.

An “aryl” group refers to an aromatic group which has at least one ringhaving a conjugated π electron system and includes carbocyclic aryl,heterocyclic aryl and biaryl groups, all of which may be optionallysubstituted. The preferred substituent(s) on aryl groups are halogen,trihalomethyl, hydroxyl, SH, cyano, alkoxy, alkyl, alkenyl, alkynyl, andamino groups.

An “alkylaryl” group refers to an alkyl group (as described above)covalently joined to an aryl group (as described above).

“Carbocyclic aryl” groups are groups wherein the ring atoms on thearomatic ring are all carbon atoms. The carbon atoms are optionallysubstituted.

“Heterocyclic aryl” groups are groups having from 1 to 3 heteroatoms asring atoms in the aromatic ring and the remainder of the ring atoms arecarbon atoms. Suitable heteroatoms include oxygen, sulfur, and nitrogen,and include furanyl, thienyl, pyridyl, pyrrolyl, pyrrolo, pyrimidyl,pyrazinyl, imidazolyl and the like, all optionally substituted.

An “amide” refers to an —C(O)—NH—R, where R is either alkyl, aryl,alkylaryl or hydrogen.

An “ester” refers to an —C(O)—OR′, where R is either alkyl, aryl, oralkylaryl.

A “blocking group” is a group which is able to be removed afterpolynucleotide synthesis and/or which is compatible with solid phasepolynucleotide synthesis.

A “phosphorus containing group” can include phosphorus in forms such asdithioates, phosphoramidites and/or as part of an oligonucleotide.

In a preferred embodiment, the invention features a process forsynthesis of the compounds of formula I.

In a preferred embodiment the invention features a process for thesynthesis of a xylofuranosyl nucleoside phosphoramidite comprising thesteps of: a) oxidation of a 2′ and 5′-protected ribonucleoside with a anoxidant such as chromium oxide/pyridine/aceticanhydride,dimethylsulfoxide/aceticanhydride, or Dess-Martin reagent (periodinane)followed by reduction with a reducing agent such as, triacetoxy sodiumborohydride, sodium borohydride, or lithium borohydride, underconditions suitable for the formation of 2′ and 5′-protectedxylofuranosyl nucleoside; b) phosphitylation under conditions suitablefor the formation of xylofuranosyl nucleoside phosphoramidite.

In yet another preferred embodiment, the invention features theincorporation of the compounds of Formula I into polynucleotides. Thesecompounds can be incorporated into polynucleotides enzymatically. Forexample by using bacteriophage T7 RNA polymerase, these novel nucleotideanalogs can be incorporated into RNA at one or more positions (Milliganet al., 1989, Methods Enzymol., 180, 51). Alternatively, novelnucleoside analogs can be incorporated into polynucleotides using solidphase synthesis (Brown and Brown, 1991, in Oligonucleotides andAnalogues: A Practical Approach, p. 1, ed. F. Eckstein, OxfordUniversity Press, New York; Wincott et al., 1995, Nucleic Acids Res.,23, 2677; Beaucage & Caruthers, 1996, in Bioorganic Chemistry: NucleicAcids, p 36, ed. S. M. Hecht, Oxford University Press, New York).

The compounds of Formula I can be used for chemical synthesis ofnucleotide-tri-phosphates and/or phosphoramidites as building blocks forselective incorporation into oligonucleotides. These oligonucleotidescan be used as an antisense molecule, 2-5A antisense chimera, triplexforming oligonucleotides (TFO) or as an enzymatic nucleic acid molecule.The oligonucleotides can also be used as probes or primers for synthesisand/or sequencing of RNA or DNA.

The compounds of the instant invention can be readily converted intonucleotide diphosphate and nucleotide triphosphates using standardprotocols (for a review see Hutchinson, 1991, in Chemistty ofNucleosides and Nucleotides, v.2, pp 81-160, Ed. L. B. Townsend, PlenumPress, New York, USA; incorporated by reference herein).

The compounds of Formula I can also be independently or in combinationused as an antiviral, anticancer or an antitumor agent. These compoundscan also be independently or in combination used with other antiviral,anticancer or an antitumor agents.

By “antisense” it is meant a non-enzymatic nucleic acid molecule thatbinds to target RNA by means of RNA-RNA or RNA-DNA or RNA-PNA (proteinnucleic acid; Egholm et al., 1993 Nature 365, 566) interactions andalters the activity of the target RNA (for a review see Stein and Cheng,1993 Science 261, 1004).

By “2-5A antisense chimera” it is meant, an antisense oligonucleotidecontaining a 5′ phosphorylated 2′-5′-linked adenylate residues. Thesechimeras bind to target RNA in a sequence-specific manner and activate acellular 2-5A-dependent ribonuclease which, in turn, cleaves the targetRNA (Torrence et al., 1993 Proc. Natl. Acad. Sci. USA 90, 1300).

By “triplex forming oligonucleotides (TFO)” it is meant anoligonucleotide that can bind to a double-stranded DNA in asequence-specific manner to form a triple-strand helix. Formation ofsuch triple helix structure has been shown to inhibit transcription ofthe targeted gene (Duval-Valentin et al., 1992 Proc. Natl. Acad. Sci.USA 89, 504).

By “enzymatic nucleic acid” it is meant a nucleic acid molecule capableof catalyzing reactions including, but not limited to, site-specificcleavage and/or ligation of other nucleic acid molecules, cleavage ofpeptide and amide bonds, and trans-splicing. Such a molecule withendonuclease activity may have complementarity in a substrate bindingregion to a specified gene target, and also has an enzymatic activitythat specifically cleaves RNA or DNA in that target. That is, thenucleic acid molecule with endonuclease activity is able tointramolecularly or intermolecularly cleave RNA or DNA and therebyinactivate a target RNA or DNA molecule. This complementarity functionsto allow sufficient hybridization of the enzymatic RNA molecule to thetarget RNA or DNA to allow the cleavage to occur. 100% complementarityis preferred, but complementarity as low as 50-75% may also be useful inthis invention. The nucleic acids may be modified at the base, sugar,and/or phosphate groups. The term enzymatic nucleic acid is usedinterchangeably with phrases such as ribozymes, catalytic RNA, enzymaticRNA, catalytic DNA, catalytic oligonucleotides, nucleozyme, DNAzyme, RNAenzyme, endoribonuclease, endonuclease, minizyme, leadzyme, oligozyme orDNA enzyme. AU of these terminologies describe nucleic acid moleculeswith enzymatic activity. The specific enzymatic nucleic acid moleculesdescribed in the instant application are not limiting in the inventionand those skilled in the art will recognize that all that is importantin an enzymatic nucleic acid molecule of this invention is that it has aspecific substrate binding site which is complementary to one or more ofthe target nucleic acid regions, and that it have nucleotide sequenceswithin or surrounding that substrate binding site which impart a nucleicacid cleaving activity to the molecule. The sites in such nucleic acidthat can be modified with nucleotides described herein are known in theart, or standard methods can be used to determine useful such sites inother such nucleic acids. See, e.g., Usman, supra.

By “enzymatic portion” or “catalytic domain” is meant thatportion/region of the ribozyme essential for cleavage of a nucleic acidsubstrate (for example see FIG. 7).

By “substrate binding arm” or “substrate binding domain” is meant thatportion/region of a ribozyme which is complementary to (i.e., able tobase-pair with) a portion of its substrate. Generally, suchcomplementarity is 100%, but can be less if desired. For example, as fewas 10 bases out of 14 may be base-paired. Such arms are shown generallyin FIGS. 1 and 3. That is, these arms contain sequences within aribozyme which are intended to bring ribozyme and target togetherthrough complementary base-pairing interactions. The ribozyme of theinvention may have binding arms that are contiguous or non-contiguousand may be varying lengths. The length of the binding arm(s) arepreferably greater than or equal to four nucleotides; specifically12-100 nucleotides; more specifically 14-24 nucleotides long. If aribozyme with two binding arms are chosen, then the length of thebinding arms are symmetrical (ie., each of the binding arms is of thesame length; e.g., six and six nucleotides or seven and sevennucleotides long) or asymmetrical (i.e., the binding arms are ofdifferent length; e.g., six and three nucleotides or three and sixnucleotides long).

In a preferred embodiment, a polynucleotide of the invention would bearone or more 2′-hydroxylamino functionalities attached directly to themonomeric unit or through the use of an appropriate spacer. Sinceoligonucleotides have neither aldehyde nor hydroxylamino groups, theformation of an oxime would occur selectively using an oligo as apolymeric template. This approach would facilitate the attachment ofpractically any molecule of interest (peptides, polyamines, coenzymes,oligosaccharides, lipids, etc.) directly to the oligonucleotide usingeither aldehyde or carboxylic function in the molecule of interest.

Advantages of Oxime Bond Formation:

The oximation reaction proceeds in water

Quantitative yields

Hydrolytic stability in a wide pH range (5-8)

The amphoteric nature of oximes allows them to act either as weak acidsor weak bases.

Oximes exhibit a great tendency to complex with metal ions

In yet another preferred embodiment, the aminooxy “tether” inoligonucleotides, such as a ribozyme, is reacted with differentcompounds bearing carboxylic groups (e.g. aminoacids, peptides, “cap”structures, etc.) resulting in the formation of oxyamides as shownbelow.

Other features and advantages of the invention will be apparent from thefollowing description of the preferred embodiments thereof, and from theclaims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The drawings will first briefly be described.

DRAWINGS

FIG. 1 shows the secondary structure model for seven different classesof enzymatic nucleic acid molecules. Arrow indicates the site ofcleavage. - - - indicate the target sequence. Lines interspersed withdots are meant to indicate tertiary interactions.—is meant to indicatebase-paired interaction. Group I Intron: P1-P9.0 represent variousstem-loop structures (Cech et al, 1994, Nature Struc. Bio., 1, 273).RNase P (M1RNA): EGS represents external guide sequence (Forster et al.,1990, Science, 249, 783; Pace et al., 1990, J. Biol. Chem., 265, 3587).Group II Intron: 5′SS means 5′ splice site; 3′SS means 3′-splice site;IBS means intron binding site; EBS means exon binding site (Pyle et al,1994, Biochemistry, 33, 2716). VS RNA: I-VI are meant to indicate sixstem-loop structures; shaded regions are meant to indicate tertiaryinteraction (Collins, International PCT Publication No. WO 96/19577).HDV Ribozyme::I-IV are meant to indicate four stem-loop structures (Beenet al, U.S. Pat. No. 5,625,047). Hammerhead Ribozyme:: I-III are meantto indicate three stem-loop structures; stems I-III can be of any lengthand may be symmetrical or asymmetrical (Usman et al., 1996, Curr. Op.Struct. Bio., 1, 527). Hairpin Ribozyme: Helix 1, 4 and 5 can be of anylength; Helix 2 is between 3 and 8 base-pairs long; Y is a pyrimidine;Helix 2 (H2) is provided with a least 4 base pairs (i.e., n is 1, 2, 3or 4) and helix 5 can be optionally provided of length 2 or more bases(preferably 3-20 bases, i.e., m is from 1-20 or more). Helix 2 and helix5 may be covalently linked by one or more bases (i.e., r is ≧1 base).Helix 1, 4 or 5 may also be extended by 2 or more base pairs (e.g., 4-20base pairs) to stabilize the ribozyme structure, and preferably is aprotein binding site. In each instance, each N and N′ independently isany normal or modified base and each dash represents a potentialbase-pairing interaction. These nucleotides may be modified at thesugar, base or phosphate. Complete base-pairing is not required in thehelices, but is preferred. Helix 1 and 4 can be of any size (i.e., o andp is each independently from 0 to any number, e.g., 20) as long as somebase-pairing is maintained. Essential bases are shown as specific basesin the structure, but those in the art will recognize that one or moremay be modified chemically (abasic, base, sugar and/or phosphatemodifications) or replaced with another base without significant effect.Helix 4 can be formed from two separate molecules, i.e., without aconnecting loop. The connecting loop when present may be aribonucleotide with or without modifications to its base, sugar orphosphate. “q” is ≧2 bases. The connecting loop can also be replacedwith a non-nucleotide linker molecule. H refers to bases A, U, or C. Yrefers to pyrimidine bases. “_(———)” refers to a covalent bond. (Burkeet al., 1996, Nucleic Acids & Mol. Biol., 10, 129; Chowrira et al, U.S.Pat. No. 5,631,359).

FIG. 2 depicts a scheme for the synthesis of a xylo ribonucleosidephosphoramidite.

FIG. 3 is a diagrammatic representation of hammerhead (HH) ribozymetargeted against stromelysin RNA (site 617) with various modifications.

Synthesis of Polynucleotides

Synthesis of polynucleotides greater than 100 nucleotides in length isdifficult using automated methods, and the therapeutic cost of suchmolecules is prohibitive. In this invention, small enzymatic nucleicacid motifs (e.g., of the hammerhead or the hairpin structure) are usedfor exogenous delivery. The simple structure of these moleculesincreases the ability of the enzymatic nucleic acid to invade targetedregions of the mRNA structure.

By “polynucleotide” as used herein is meant a molecule having two ormore nucleotides. The polynucleotide can be single, double or multiplestranded and may comprise modified or unmodified nucleotides ornon-nucleotides or various mixtures and combinations thereof.

RNA molecules, such as the ribozymes are chemically synthesized. Themethod of synthesis used follows the procedure for normal RNA synthesisas described in Usman et al., 1987 J. Am. Chem. Soc., 109, 7845;Scaringe et al., 1990 Nucleic Acids Res., 18, 5433; and Wincott et al.,1995 Nucleic Acids Res. 23, 2677-2684 and makes use of common nucleicacid protecting and coupling groups, such as dimethoxytrityl at the5′-end, and phosphoramidites at the 3′-end. Small scale synthesis wereconducted on a 394 Applied Biosystems, Inc. synthesizer using a modified2.5 μmol scale protocol with a 5 min coupling step for alkylsilylprotected nucleotides and 2.5 min coupling step for 2′-O-methylatednucleotides. Table II outlines the amounts, and the contact times, ofthe reagents used in the synthesis cycle. A 6.5-fold excess (163 μL of0.1 M=16.3 μmol) of phosphoramidite and a 24-fold excess of S-ethyltetrazole (238 μL of 0.25 M=59.5 μmol) relative to polymer-bound5′-hydroxyl was used in each coupling cycle. Average coupling yields onthe 394 Applied Biosystems, Inc. synthesizer, determined by colorimetricquantitation of the trityl fractions, were 97.5-99%. Otheroligonucleotide synthesis reagents for the 394 Applied Biosystems, Inc.synthesizer: detritylation solution was 2% TCA in methylene chloride(ABI); capping was performed with 16% N-methyl imidazole in THF (ABI)and 10% acetic anhydride/10% 2,6-lutidine in THF (ABI); oxidationsolution was 16.9 mM 12, 49 mM pyridine, 9% water in THF (Millipore). B& J Synthesis Grade acetonitrile was used directly from the reagentbottle. S-Ethyl tetrazole solution (0.25 M in acetonitrile) was made upfrom the solid obtained from American International Chemical, Inc.

Deprotection of the RNA was performed as follows. The polymer-boundoligoribonucleotide, trityl-off, was transferred from the synthesiscolumn to a 4 mL glass screw top vial and suspended in a solution ofmethylamine (MA) at 65° C. for 10 min. After cooling to −20° C., thesupernatant was removed from the polymer support. The support was washedthree times with 1.0 mL of EtOH:MeCN:H₂O/3:1:1, vortexed and thesupernatant was then added to the first supernatant. The combinedsupernatants, containing the oligoribonucleotide, were dried to a whitepowder.

The base-deprotected oligoribonucleotide was resuspended in anhydrousTEA•HF/NMP solution (250 μL of a solution of 1.5 mLN-methylpyrrolidinone, 750 μL TEA and 1.0 mL TEA•3HF to provide a 1.4MHF concentration) and heated to 65° C. for 1.5 h. The resulting, fullydeprotected, oligomer was quenched with 50 mM TEAB (9 mL) prior to anionexchange desalting.

For anion exchange desalting of the deprotected oligomer, the TEABsolution was loaded onto a Qiagen 500® anion exchange cartridge (QiagenInc.) that was prewashed with 50 mM TEAB (10 mL). After washing theloaded cartridge with 50 mM TEAB (10 mL), the RNA was eluted with 2 MTEAB (10 mL) and dried down to a white powder.

RNAs are purified by gel electrophoresis using general methods or arepurified by high pressure liquid chromatography (HPLC; See Stinchcomb etal., International PCT Publication No. WO 95/23225, the totality ofwhich is hereby incorporated herein by reference) and are resuspended inwater.

Enzymatic Nucleic Acid Molecules:

The enzymatic nucleic acid is able to intramolecularly orintermolecularly cleave RNA or DNA and thereby inactivate a target RNAor DNA molecule. The enzymatic nucleic acid molecule that hascomplementarity in a substrate binding region to a specified genetarget, also has an enzymatic activity that specifically cleaves RNA orDNA in that target. This complementarity functions to allow sufficienthybridization of the enzymatic RNA molecule to the target RNA or DNA toallow the cleavage to occur. 100% Complementarity is preferred, butcomplementarity as low as 50-75% may also be useful in this invention.The nucleic acids may be modified at the base, sugar, and/or phosphategroups.

The term enzymatic nucleic acid is used interchangeably with phrasessuch as ribozymes, catalytic RNA, enzymatic RNA, catalytic DNA,nucleozyme, DNAzyme, RNA enzyme, endoribonuclease, minizyme, leadzyme,oligozyme, or DNA enzyme.

By “complementarity” is meant a nucleic acid that can form hydrogenbond(s) with other RNA sequence by either traditional Watson-Crick orother nontraditional types (for example, Hoogsteen type) of base-pairedinteractions.

Nucleic acid molecules having an endonuclease enzymatic activity areable to repeatedly cleave other separate RNA molecules in a nucleotidebase sequence-specific manner. Such enzymatic RNA molecules can betargeted to virtually any RNA transcript, and efficient cleavageachieved in vitro (Zaug et al., 324, Nature 429 1986; Kim et al., 84Proc. Natl. Acad. Sci. USA 8788, 1987; Haseloff and Gerlach, 334 Nature585, 1988; Cech, 260 JAMA 3030, 1988; and Jefferies et al., 17 NucleicAcids Research 1371, 1989).

Because of their sequence-specificity, trans-cleaving ribozymes showpromise as therapeutic agents for human disease (Usman & McSwiggen, 1995Ann. Rep. Med. Chem. 30, 285-294; Christoffersen and Marr, 1995 J. Med.Chem. 38, 2023-2037). Ribozymes can be designed to cleave specific RNAtargets within the background of cellular RNA. Such a cleavage eventrenders the mRNA non-functional and abrogates protein expression fromthat RNA. In this manner, synthesis of a protein associated with adisease state can be selectively inhibited.

Seven basic varieties of naturally-occurring enzymatic RNAs are knownpresently. Each can catalyze the hydrolysis of RNA phosphodiester bondsin trans (and thus can cleave other RNA molecules) under physiologicalconditions. Table I summarizes some of the characteristics of theseribozymes. In general, enzymatic nucleic acids act by first binding to atarget RNA. Such binding occurs through the target binding portion of aenzymatic nucleic acid which is held in close proximity to an enzymaticportion of the molecule that acts to cleave the target RNA. Thus, theenzymatic nucleic acid first recognizes and then binds a target RNAthrough complementary base-pairing, and once bound to the correct site,acts enzymatically to cut the target RNA. Strategic cleavage of such atarget RNA will destroy its ability to direct synthesis of an encodedprotein. After an enzymatic nucleic acid has bound and cleaved its RNAtarget, it is released from that RNA to search for another target andcan repeatedly bind and cleave new targets.

The enzymatic nature of a ribozyme is advantageous over othertechnologies, since the effective concentration of ribozyme necessary toeffect a therapeutic treatment is lower than that of an antisenseoligonucleotide. This advantage reflects the ability of the ribozyme toact enzymatically. Thus, a single ribozyme molecule is able to cleavemany molecules of target RNA. In addition, the ribozyme is a highlyspecific inhibitor, with the specificity of inhibition depending notonly on the base-pairing mechanism of binding, but also on the mechanismby which the molecule inhibits the expression of the RNA to which itbinds. That is, the inhibition is caused by cleavage of the RNA targetand so specificity is defined as the ratio of the rate of cleavage ofthe targeted RNA over the rate of cleavage of non-targeted RNA. Thiscleavage mechanism is dependent upon factors additional to thoseinvolved in base-pairing. Thus, it is thought that the specificity ofaction of a ribozyme is greater than that of antisense oligonucleotidebinding the same RNA site.

In one aspect, an enzymatic nucleic acid molecule is formed in ahammerhead (see for example FIGS. 1 and 2) or hairpin motif (FIG. 1),but may also be formed in the motif of a hepatitis delta virus (HDV),group I intron, RNaseP RNA (in association with an external guidesequence) or Neurospora VS RNA (FIG. 1). Examples of such hammerheadmotifs are described by Rossi et al., 1992, Aids Research and HumanRetroviruses 8, 183; Usman et al., 1996, Curr. Op. Struct. Biol., 1,527; of hairpin motifs by Hampel et al., EP 0360257; Hampel and Tritz,1989 Biochemistry 28, 4929; and Hampel et al., 1990 Nucleic Acids Res.18, 299; Chowrira et al., U.S. Pat. No. 5,631,359; an example of thehepatitis delta virus motif is described by Perrotta and Been, 1992Biochemistry 31, 16; Been et al., U.S. Pat. No. 5,625,047; of the RNasePmotif by Guerrier-Takada et al., 1983 Cell 35, 849; Forster and Altman,1990 Science 249, 783; Neurospora VS RNA ribozyme motif is described byCollins (Saville and Collins, 1990 Cell 61, 685-696; Saville andCollins, 1991 Proc. Natl. Acad. Sci. USA 88, 8826-8830; Guo and Collins,1995 EMBO J. 14, 368) and of the Group I intron by Zaug et at, 1986,Nature, 324, 429; Cech et al., U.S. Pat. No. 4,987,071. (All thesepublications are hereby incorporated by references herein.) Thesespecific motifs are not limiting in the invention and those skilled inthe art will recognize that all that is important in an enzymaticnucleic acid molecule with endonuclease activity of this invention isthat it has a specific substrate binding site which is complementary toone or more of the target gene RNA and that it have nucleotide sequenceswithin or surrounding that substrate binding site which impart an RNAcleaving activity to the molecule. The length of the binding site variesfor different ribozyme motifs, and a person skilled in the art willrecognize that to achieve an optimal ribozyme activity the length of thebinding arm should be of sufficient length to form a stable interactionwith the target nucleic acid sequence.

Catalytic activity of the ribozymes described in the instant inventioncan be optimized as described by Draper et al., supra. The details willnot be repeated here, but include altering the length of the ribozymebinding arms, or chemically synthesizing ribozymes with modifications(base, sugar and/or phosphate) that prevent their degradation by serumribonucleases and/or enhance their enzymatic activity (see e.g.,Eckstein et al., International Publication No. WO 92/07065; Perrault etal., 1990 Nature 344, 565; Pieken et al., 1991 Science 253, 314; Usmanand Cedergren, 1992 Trends in Biochem Sci. 17, 334; Usman et al.,International Publication No. WO 93/15187; and Rossi et al.,International Publication No. WO 91/03162; Sproat, U.S. Pat. No.5,334,711; and Burgin et al., supra; all of these describe variouschemical modifications that can be made to the base, phosphate and/orsugar moieties of enzymatic RNA molecules). Modifications which enhancetheir efficacy in cells, and removal of bases from stem loop structuresto shorten RNA synthesis times and reduce chemical requirements aredesired. (All these publications are hereby incorporated by referenceherein).

There are several examples in the art describing sugar and phosphatemodifications that can be introduced into enzymatic nucleic acidmolecules without significantly effecting catalysis and with significantenhancement in their nuclease stability and efficacy. Ribozymes aremodified to enhance stability and/or enhance catalytic activity bymodification with nuclease resistant groups, for example, 2′-amino,2′-C-allyl, 2′-flouro, 2′-O-methyl, 2′-H, nucleotide base modifications(for a review see Usman and Cedergren, 1992 TIBS 17, 34; Usman et al.,1994 Nucleic Acids Symp. Ser. 31, 163; Burgin et al, 1996 Biochemistry35, 14090). Sugar modification of enzymatic nucleic acid molecules havebeen extensively described in the art (see Eckstein et al.,International Publication PCT No. WO 92/07065; Perrault et al Nature1990, 344, 565-568; Pieken et al. Science 1991, 253, 314-317; Usman andCedergren, Trends in Biochem. Sci. 1992, 17, 334-339; Usman et al.International Publication PCT No. WO 93/15187; Sproat, U.S. Pat. No.5,334,711 and Beigelman et al., 1995 J. Biol. Chem. 270, 25702; all ofthe references are hereby incorporated in their totality by referenceherein).

Such publications describe general methods and strategies to determinethe location of incorporation of sugar, base and/or phosphatemodifications and the like into ribozymes without inhibiting catalysis,and are incorporated by reference herein. In view of such teachings,similar modifications can be used as described herein to modify thenucleic acid catalysts of the instant invention.

Nucleic acid catalysts having chemical modifications which maintain orenhance enzymatic activity are provided. Such nucleic acid is alsogenerally more resistant to nucleases than unmodified nucleic acid.Thus, in a cell and/or in vivo the activity may not be significantlylowered. As exemplified herein such ribozymes are useful in a celland/or in vivo even if activity over all is reduced 10 fold (Burgin etal., 1996, Biochemistry, 35, 14090). Such ribozymes herein are said to“maintain” the enzymatic activity on all RNA ribozyme.

Therapeutic ribozymes delivered exogenously must optimally be stablewithin cells until translation of the target RNA has been inhibited longenough to reduce the levels of the undesirable protein. This period oftime varies between hours to days depending upon the disease state.Clearly, ribozymes must be resistant to nucleases in order to functionas effective intracellular therapeutic agents. Improvements in thechemical synthesis of RNA (Wincott et al., 1995 Nucleic Acids Res. 23,2677; incorporated by reference herein) have expanded the ability tomodify ribozymes by introducing nucleotide modifications to enhancetheir nuclease stability as described above.

By “nucleotide” as used herein is as recognized in the art to includenatural bases (standard), and modified bases well known in the art. Suchbases are generally located at the 1′ position of a sugar moiety.Nucleotide generally comprise a base, sugar and a phosphate group. Thenucleotides can be unmodified or modified at the sugar, phosphate and/orbase moiety, (also referred to interchangeably as nucleotide analogs,modified nucleotides, non-natural nucleotides, non-standard nucleotidesand other; see for example, Usman and McSwiggen, supra; Eckstein et al.,International PCT Publication No. WO 92/07065; Usman et al.,International PCT Publication No. WO 93/15187; all hereby incorporatedby reference herein). There are several examples of modified nucleicacid bases known in the art and has recently been summarized by Limbachet al., 1994, Nucleic Acids Res. 22, 2183. Some of the non-limitingexamples of base modifications that can be introduced into enzymaticnucleic acids without significantly effecting their catalytic activityinclude, inosine, purine, pyridin-4-one, pyridin-2-one, phenyl,pseudouracil, 2,4,6-trimethoxy benzene, 3-methyl uracil, dihydrouridine,naphthyl, aminophenyl, 5-alkylcytidines (e.g., 5-methylcytidine),5-alkyluridines (e.g., ribothymidine), 5-halouridine (e.g.,5-bromouridine) or 6-azapyrimidines or 6-alkylpyrimidines (e.g.6-methyluridine) and others (Burgin et al., 1996, Biochemistry, 35,14090). By “modified bases” in this aspect is meant nucleotide basesother than adenine, guanine, cytosine and uracil at 1′ position or theirequivalents; such bases may be used within the catalytic core of theenzyme and/or in the substrate-binding regions.

By “unmodified nucleoside” is meant one of the bases adenine, cytosine,guanine, uracil joined to the 1′ carbon of b-D-ribo-furanose.

By “modified nucleoside” is meant any nucleotide base which contains amodification in the chemical structure of an unmodified nucleotide base,sugar and/or phosphate.

Various modifications to ribozyme structure can be made to enhance theutility of ribozymes. Such modifications will enhance shelf-life,half-life in vitro, stability, and ease of introduction of suchribozymes to the target site, e.g., to enhance penetration of cellularmembranes, and confer the ability to recognize and bind to targetedcells.

Administration of Polynucleotides

Sullivan et al., PCT WO 94/02595, describes the general methods fordelivery of enzymatic RNA molecules. Ribozymes may be administered tocells by a variety of methods known to those familiar to the art,including, but not restricted to, encapsulation in liposomes, byiontophoresis, or by incorporation into other vehicles, such ashydrogels, cyclodextrins, biodegradable nanocapsules, and bioadhesivemicrospheres. For some indications, ribozymes may be directly deliveredex vivo to cells or tissues with or without the aforementioned vehicles.Alternatively, the RNA/vehicle combination is locally delivered bydirect injection or by use of a catheter, infusion pump or stent. Otherroutes of delivery include, but are not limited to, intravascular,intramuscular, subcutaneous or joint injection, aerosol inhalation, oral(tablet or pill form), topical, systemic, ocular, intraperitoneal and/orintrathecal delivery. More detailed descriptions of ribozyme deliveryand administration are provided in Sullivan et al., supra and Draper etal., PCT WO93/23569 which have been incorporated by reference herein.

The molecules of the instant invention can be used as pharmaceuticalagents. Pharmaceutical agents prevent, inhibit the occurrence, or treat(alleviate a symptom to some extent, preferably all of the symptoms) ofa disease state in a patient.

By “patient” is meant an organism which is a donor or recipient ofexplanted cells or the cells themselves. “Patient” also refers to anorganism to which the compounds of the invention can be administered.Preferably, a patient is a mammal, e.g., a human, primate or a rodent.

The negatively charged polynucleotides of the invention can beadministered (e.g., RNA, DNA or protein) and introduced into a patientby any standard means, with or without stabilizers, buffers, and thelike, to form a pharmaceutical composition. When it is desired to use aliposome delivery mechanism, standard protocols for formation ofliposomes can be followed. The compositions of the present invention mayalso be formulated and used as tablets, capsules or elixirs for oraladministration; suppositories for rectal administration; sterilesolutions; suspensions for injectable administration; and the like.

The present invention also includes pharmaceutically acceptableformulations of the compounds described. These formulations includesalts of the above compounds, e.g., acid addition salts, for example,salts of hydrochloric, hydrobromic, acetic acid, and benzene sulfonicacid.

A pharmacological composition or formulation refers to a composition orformulation in a form suitable for administration, e.g., systemicadministration, into a cell or patient, preferably a human. Suitableforms, in part, depend upon the use or the route of entry, for exampleoral, transdermal, or by injection. Such forms should not prevent thecomposition or formulation to reach a target cell (ie., a cell to whichthe negatively charged polymer is desired to be delivered to). Forexample, pharmacological compositions injected into the blood streamshould be soluble. Other factors are known in the art, and includeconsiderations such as toxicity and forms which prevent the compositionor formulation from exerting its effect.

By “systemic administration” is meant in vivo systemic absorption oraccumulation of drugs in the blood stream followed by distributionthroughout the entire body. Administration routes which lead to systemicabsorption include, without limitations: intravenous, subcutaneous,intraperitoneal, inhalation, oral, intrapulmonary and intramuscular.Each of these administration routes expose the desired negativelycharged polymers, e.g., nucleic acids, to an accessible diseased tissue.The rate of entry of a drug into the circulation has been shown to be afunction of molecular weight or size. The use of a liposome or otherdrug carrier comprising the compounds of the instant invention canpotentially localize the drug, for example, in certain tissue types,such as the tissues of the reticular endothelial system (RES). Aliposome formulation which can facilitate the association of drug withthe surface of cells, such as, lymphocytes and macrophages is alsouseful. This approach may provide enhanced delivery of the drug totarget cells by taking advantage of the specificity of macrophage andlymphocyte immune recognition of abnormal cells, such as the cancercells.

The invention also features the use of the a composition comprisingsurface-modified liposomes containing poly (ethylene glycol) lipids(PEG-modified, or long-circulating liposomes or stealth liposomes).These formulations offer an method for increasing the accumulation ofdrugs in target tissues. This class of drug carriers resistsopsonization and elimination by the mononuclear phagocytic system (MPSor RES), thereby enabling longer blood circulation times and enhancedtissue exposure for the encapsulated drug (Lasic et al. Chem. Rev. 1995,95, 2601-2627; Ishiwata et al., Chem. Pharm. Bull. 1995, 43, 1005-1011).Such liposomes have been shown to accumulate selectively in tumors,presumably by extravasation and capture in the neovascularized targettissues (Lasic et al, Science 1995, 267, 1275-1276; Oku et al., 1995,Biochim. Biophys. Acta, 1238, 86-90). The long-circulating liposomesenhance the pharmacokinetics and pharmacodynamics of DNA and RNA,particularly compared to conventional cationic liposomes which are knownto accumulate in tissues of the MPS (Liu et al., J. Biol. Chem. 1995,42, 24864-24870; Choi et al., International PCT Publication No. WO96/10391; Ansell et al, International PCT Publication No. WO 96/10390;Holland et al., International PCT Publication No. WO 96/10392; all ofthese are incorporated by reference herein). Long-circulating liposomesare also likely to protect drugs from nuclease degradation to a greaterextent compared to cationic liposomes, based on their ability to avoidaccumulation in metabolically aggressive MPS tissues such as the liverand spleen. All of these references are incorporated by referenceherein.

The present invention also includes compositions prepared for storage oradministration which include a pharmaceutically effective amount of thedesired compounds in a pharmaceutically acceptable carrier or diluent.Acceptable carriers or diluents for therapeutic use are well known inthe pharmaceutical art, and are described, for example, in Remington'sPharmaceutical Sciences, Mack Publishing Co. (A. R. Gennaro edit. 1985)hereby incorporated by reference herein. For example, preservatives,stabilizers, dyes and flavoring agents may be provided. Id. at 1449.These include sodium benzoate, sorbic acid and esters ofp-hydroxybenzoic acid. In addition, antioxidants and suspending agentsmay be used. Id

A pharmaceutically effective dose is that dose required to prevent,inhibit the occurrence, or treat (alleviate a symptom to some extent,preferably all of the symptoms) of a disease state. The pharmaceuticallyeffective dose depends on the type of disease, the composition used, theroute of administration, the type of mammal being treated, the physicalcharacteristics of the specific mammal under consideration, concurrentmedication, and other factors which those skilled in the medical artswill recognize. Generally, an amount between 0.1 mg/kg and 100 mg/kgbody weight/day of active ingredients is administered dependent uponpotency of the negatively charged polymer.

EXAMPLES

The following are non-limiting examples showing the synthesis andactivity of the certain compounds of Formula I of the instant inventionand polynucleotides comprising one or more of these compounds. Those inthe art will recognize that certain reaction conditions such astemperatures, pH, ionic conditions, reaction times and solventconditions described in the following examples are not meant to belimiting and can be readily modified without significantly effecting thesynthesis.

Example 1 Synthesis of Xylo Nucleoside Phosphoramidite

Preparation of Protected 1-(β-D-xylopentofuranosyl)nucleoside:

Referring to FIG. 2, 2′-O-TBDMS-5′-O-DMT ribonucleoside 1 (2 mmol) wasadded to a solution of CrO₃ (600 mg), pyridine (1 ml) and aceticanhydride (0.6 ml) in CH₂Cl₂ (15 ml) and the reaction mixture stirred atroom temperature for 1 hour. Ethyl acetate (100 ml) was then added andthe mixture filtered through a Celite pad. The filtrate was concentratedin vacuo (40° C.), ethyl acetate (100 ml) was added and the mixturefiltered slowly through the mixture of silica gel and Florisil (1:1, 40g). The filtrate was concentrated in vacuo (40° C.) and used directly inthe next step.

The above material was dissolved in ethanol (30 ml) and NaB(OAc)₃H (848mg, 2 eq) was added. The reaction mixture was stirred at roomtemperature overnight and the solvent removed in vacuo. The residue waspartitioned between ethyl acetate and brine, organic layer was washedwith aqueous 5% NaHCO₃ solution, dried (Na₂SO₄) and evaporated to acolorless foam. Purification by flash silica gel column chromatographyusing CH₂Cl₂/MeOH or CH₂Cl₂/THF mixtures yielded pure products (scheme1, 2) in 60-75% yield (based on the starting ribo nucleosides).

Preparation of 3′-O-phosphoramidites:

3′-O-Phosphoramidites 3 were prepared in 65-70(G) or-75-85(A) % yieldusing the standard phosphitylation procedure (Tuschl, T., et al.Biochemistry 1993, 32, 11658-11668).

This scheme can be used to synthesize xylo nucleoside phosphoramiditessuch as xyloadenosine, xyloguanosine, xylouridine, xylocytidine andothers.

Example 2 Incorporation of Phosphoramidites into Ribozymes

The above monomers 3 were incorporated into ribozymes using standardprocedures (Wincott, et al. Nucleic Acids Res 1995, 23, 2677-2684; Usmanet al., J. Am. Chem. Soc. 1987, 109, 7845-7854; Scaringe et al., NucleicAcids Res. 1990, 18, 5433-5441) and makes use of common nucleic acidprotecting and coupling groups, such as dimethoxytrityl at the 5′-end,and phosphoramidites at the 3′-end. The average stepwise coupling yieldswere >98%. These nucleotides may be incorporated not only intohammerhead ribozymes, but also into hairpin, VS ribozymes, hepatitisdelta virus, or Group I or Group II introns. They are, therefore, ofgeneral use as replacement motifs in any nucleic acid structure. Thecoupling time for the incorporation of modified phosphoramidites wasextended to 20 minutes (Seela, F., et al. 1 Helv. Chim. Acta 1994, 77,883-895). Examples of ribozyme synthesized according to this inventionare shown in FIG. 3.

Example 3 Cleavage of Short Substrate Using Xylo Modified Ribozymes

Ribozyme Reactions:

Ribozyme (1 μM) was incubated in 50 mM Tris (pH 8.0) and 10 mM MgCl₂ at37° C. with trace amounts of short substrate (>1 nMol of RNA). Reactiontimes were modulated to give accurate kinetics of cleavage values.Ribozyme with xylo A residue at A15.1 and/or A6 demonstrated the samecleavage activity as parent “5-Ribo” motif (Table IIIA). Incorporationof xylo G at G12 is also tolerated though cleavage activity is reducedby 5 fold (Table IIIB).

Diagnostic Uses

Ribozymes of this invention may be used as diagnostic tools to examinegenetic drift and mutations within diseased cells or to detect thepresence of a specific RNA in a cell. The close relationship betweenribozyme activity and the structure of the target RNA allows thedetection of mutations in any region of the molecule which alters thebase-pairing and three-dimensional structure of the target RNA. By usingmultiple ribozymes described in this invention, one may map nucleotidechanges which are important to RNA structure and function in vitro, aswell as in cells and tissues. Cleavage of target RNAs with ribozymes maybe used to inhibit gene expression and define the role (essentially) ofspecified gene products in the progression of disease. In this manner,other genetic targets may be defined as important mediators of thedisease. These experiments will lead to better treatment of the diseaseprogression by affording the possibility of combinational therapies(e.g., multiple ribozymes targeted to different genes, ribozymes coupledwith known small molecule inhibitors, or intermittent treatment withcombinations of ribozymes and/or other chemical or biologicalmolecules). Other in vitro uses of ribozymes of this invention are wellknown in the art, and include detection of the presence of mRNAsassociated with related condition. Such RNA is detected by determiningthe presence of a cleavage product after treatment with a ribozyme usingstandard methodology.

In a specific example, ribozymes which can cleave only wild-type ormutant forms of the target RNA are used for the assay. The firstribozyme is used to identify wild-type RNA present in the sample and thesecond ribozyme will be used to identify mutant RNA in the sample. Asreaction controls, synthetic substrates of both wild-type and mutant RNAwill be cleaved by both ribozymes to demonstrate the relative ribozymeefficiencies in the reactions and the absence of cleavage of the“non-targeted” RNA species. The cleavage products from the syntheticsubstrates will also serve to generate size markers for the analysis ofwild-type and mutant RNAs in the sample population. Thus each analysiswill require two ribozymes, two substrates and one unknown sample whichwill be combined into six reactions. The presence of cleavage productswill be determined using an RNAse protection assay so that full-lengthand cleavage fragments of each RNA can be analyzed in one lane of apolyacrylamide gel. It is not absolutely required to quantify theresults to gain insight into the expression of mutant RNAs and putativerisk of the desired phenotypic changes in target cells. The expressionof mRNA whose protein product is implicated in the development of thephenotype is adequate to establish risk. If probes of comparablespecific activity are used for both transcripts, then a qualitativecomparison of RNA levels will be adequate and will decrease the cost ofthe initial diagnosis. Higher mutant form to wild-type ratios will becorrelated with higher risk whether RNA levels are comparedqualitatively or quantitatively.

Additional Uses

Nucleic acid molecules of the instant invention might have many of thesame applications for the study of RNA that DNA restrictionendonucleases have for the study of DNA (Nathans et al., 1975 Ann. Rev.Biochem. 44:273). For example, the pattern of restriction fragmentscould be used to establish sequence relationships between two relatedRNAs, and large RNAs could be specifically cleaved to fragments of asize more useful for study. The ability to engineer sequence specificityof the ribozyme is ideal for cleavage of RNAs of unknown sequence.Nucleic acid molecules (e.g., ribozymes) of the invention can be used,for example, to target cleavage of virtually any RNA transcript (Zaug etal., 324, Nature 429 1986; Cech, 260 JAMA 3030, 1988; and Jefferies etal., 17 Nucleic Acids Research 1371, 1989). Such nucleic acids can beused as a therapeutic or to validate a therapeutic gene target and/or todetermine the function of a gene in a biological system (Christoffersen,1997, Nature Biotech. 15, 483).

Various ligands can be attached to oligonucleotides using the compondsof Formula I for the purposes of cellular delivery, nuclease resistance,cellular trafficking and localization, chemical ligation ofoligonucleotide fragments. Incorporation of one or more compounds ofFormula I into a ribozyme may increase its effectiveness. Compounds ofFormula I can be used as potential antiviral agents.

Other embodiments are within the following claims.

TABLE I Characteristics of naturally occurring ribozymes Group I IntronsSize: ˜150 to >1000 nucleotides. Requires a U in the target sequenceimmediately 5′ of the cleavage site. Binds 4-6 nucleotides at the5′-side of the cleavage site. Reaction mechanism: attack by the 3′-OH ofguanosine to generate cleavage products with 3′-OH and 5′-guanosine.Additional protein cofactors required in some cases to help folding andmaintainance of the active structure. Over 300 known members of thisclass. Found as an intervening sequence in Tetrahymena thermophila rRNA,fungal mitochondria, chloro- plasts, phage T4, blue-green algae, andothers. Major structural features largely established throughphylogenetic comparisons, mutagenesis, and biochemical studies[^(i),^(ii)]. Complete kinetic framework established for one ribozyme[^(iii),^(iv),^(v),^(vi)]. Studies of ribozyme folding and substratedocking underway [^(vii),^(viii),^(ix)]. Chemical modificationinvestigation of important residues well established [^(x),^(xi)]. Thesmall (4-6 nt) binding site may make this ribozyme too non-specific fortargeted RNA cleavage, however, the Tetrahymena group I intron has beenused to repair a “defective” β-galactosidase message by the ligation ofnew β-galactosidase sequences onto the defective message [^(xii)]. RNAseP RNA (M1 RNA) Size: ˜290 to 400 nucleotides. RNA portion of aubiquitous ribonucleoprotein enzyme. Cleaves tRNA precursors to formmature tRNA [^(xiii)]. Reaction mechanism: possible attack by M²⁺-OH togenerate cleavage products with 3′-OH and 5′-phosphate. RNAse P is foundthroughout the prokaryotes and eukaryotes. The RNA subunit has beensequenced from bacteria, yeast, rodents, and primates. Recruitment ofendogenous RNAse P for therapeutic applications is pos- sible throughhybridization of an External Guide Sequence (EGS) to the target RNA[^(xiv),^(xv)] Important phosphate and 2′ OH contacts recentlyidentified [^(xvi),^(xvii)] Group II Introns Size: >1000 nucleotides.Trans cleavage of target RNAs recently demonstrated [^(xviii),^(xix)].Sequence requirements not fully determined. Reaction mechanism: 2′-OH ofan internal adenosine generates cleavage products with 3′-OH and a“lariat” RNA containing a 3′-5′ and a 2′-5′ branch point. Only naturalribozyme with demonstrated participation in DNA cleavage [^(xx),^(xxi)]in addition to RNA cleavage and ligation. Major structural featureslargely established through phylogenetic comparisons [^(xxii)].Important 2′ OH contacts beginning to be identified [^(xxiii)] Kineticframework under development [^(xxiv)] Neurospora VS RNA Size: ˜144nucleotides. Trans cleavage of hairpin target RNAs recently demonstrated[^(xxv)]. Sequence requirements not fully determined. Reactionmechanism: attack by 2′-OH 5′ to the scissile bond to generate cleavageproducts with 2′,3′-cyclic phosphate and 5′-OH ends. Binding sites andstructural requirements not fully determined. Only 1 known member ofthis class. Found in Neurospora VS RNA. Hammerhead Ribozyme (see textfor references) Size: ˜13 to 40 nucleotides. Requires the targetsequence UH immediately 5′ of the cleavage site. Binds a variable numbernucleotides on both sides of the cleavage site. Reaction mechanism:attack by 2′-OH 5′ to the scissile bond to generate cleavage productswith 2′,3′-cyclic phosphate and 5′-OH ends. 14 known members of thisclass. Found in a number of plant pathogens (virusoids) that use RNA asthe infectious agent. Essential structural features largely defined,including 2 crystal structures [^(xxvi),^(xxvii)] Minimal ligationactivity demonstrated (for engineering through in vitro selection)[^(xxviii)] Complete kinetic framework established for two or moreribozymes [^(xxix)]. Chemical modification investigation of importantresidues well established [^(xxx)]. Hairpin Ribozyme Size: ˜50nucleotides. Requires the target sequence GUC immediately 3′ of thecleavage site. Binds 4-6 nucleotides at the 5′-side of the cleavage siteand a variable number to the 3′-side of the cleavage site. Reactionmechanism: attack by 2′-OH 5′ to the scissile bond to generate cleavageproducts with 2′,3′-cyclic phosphate and 5′-OH ends. 3 known members ofthis class. Found in three plant pathogen (satellite RNAs of the tobaccoringspot virus, arabis mosaic virus and chicory yellow mottle virus)which uses RNA as the infectious agent. Essential structural featureslargely defined [^(xxxi),^(xxxii),^(xxxiii),^(xxxiv)] Ligation activity(in addition to cleavage activity) makes ribozyme amenable toengineering through in vitro selection [^(xxxv)] Complete kineticframework established for one ribozyme [^(xxxvi)]. Chemical modificationinvestigation of important residues begun [^(xxxvii),^(xxxviii)].Hepatitis Delta Virus (HDV) Ribozyme Size: ˜60 nucleotides. Transcleavage of target RNAs demonstrated [^(xxxix)]. Binding sites andstructural requirements not fully determined, although no sequences 5′of cleavage site are required. Folded ribozyme contains a pseudoknotstructure [^(xl)]. Reaction mechanism: attack by 2′-OH 5′ to thescissile bond to generate cleavage products with 2′,3′-cyclic phosphateand 5′-OH ends. Only 2 known members of this class. Found in human HDV.Circular form of HDV is active and shows increased nuclease stability[xli] ^(i).Michel, Francois; Westhof, Eric. Slippery substrates. Nat.Struct. Biol. 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A mechanistic framework for thesecond step of splicing catalyzed by the Tetrahymena ribozyme.Biochemistry (1996), 35(2), 648-58. ^(vii).Li, Yi; Bevilacqua, PhilipC.; Mathews, David; Turner, Douglas H.. Thermodynamic and activationparameters for binding of a pyrene-labeled substrate by the Tetrahymenaribozyme: docking is not diffusion-controlled and is driven by afavorable entropy change. Biochemistry (1995), 34(44), 14394-9.^(viii).Banerjee, Aloke Raj; Turner, Douglas H.. The time dependence ofchemical modification reveals slow steps in the folding of a group Iribozyme. Biochemistry (1995), 34(19), 6504-12. ^(ix).Zarrinkar, PatrickP.; Williamson, James R.. The P9.1-P9.2 peripheral extension helps guidefolding of the Tetrahymena ribozyme. Nucleic Acids Res. (1996), 24(5),854-8. ^(x).Strobel, Scott A.; Cech, Thomas R.. Minor groove recognitionof the conserved G.cntdot.U pair at the Tetrahymena ribozyme reactionsite. Science (Washington, D.C.) 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Probing of tertiary interactions in RNA: 2′-hydroxyl-base contactsbetween the RNase P RNA and pre-tRNA. Proc. Natl. Acad. Sci. U.S.A.(1995), 92(26), 12510-14. ^(xviii).Pyle, Anna Marie; Green, Justin B..Building a Kinetic Framework for Group II Intron Ribozyme Activity:Quantitation of Interdomain Binding and Reaction Rate. Biochemistry(1994), 33(9), 2716-25. ^(xix).Michels, William J. Jr.; Pyle, AnnaMarie. Conversion of a Group II Intron into a New Multiple-TurnoverRibozyme that Selectively Cleaves Oligonucleotides: Elucidation ofReaction Mechanism and Structure/Function Relationships. Biochemistry(1995), 34(9), 2965-77. ^(xx).Zimmerly, Steven; Guo, Huatao; Eskes,Robert; Yang, Jian; Perlman, Philip S.; Lambowitz, Alan M.. A group IIintron RNA is a catalytic component of a DNA endonuclease involved inintron mobility. Cell (Cambridge, Mass.) (1995), 83(4), 529-38.^(xxi).Griffin, Edmund A., Jr.; Qin, Zhifeng; Michels, Williams J., Jr.;Pyle, Anna Marie. Group II intron ribozymes that cleave DNA and RNAlinkages with similar efficiency, and lack contacts with substrate2′-hydroxyl groups. Chem. Biol. (1995), 2(11), 761-70. ^(xxii).Michel,Francois; Ferat, Jean Luc. Structure and activities of group II introns.Annu. Rev. Biochem. (1995), 64, 435-61. ^(xxiii).Abramovitz, Dana L.;Friedman, Richard A.; Pyle, Anna Marie. Catalytic role of 2′-hydroxylgroups within a group II intron active site. Science (Washington, D.C.)(1996), 271(5254), 1410-13. ^(xxiv).Daniels, Danette L.; Michels,William J., Jr.; Pyle, Anna Marie. Two competing pathways forself-splicing by group II introns: a quantitative analysis of in vitroreaction rates and products. J. Mol. Biol. (1996), 256(1), 31-49.^(xxv).Guo, Hans C. T.; Collins, Richard A.. Efficient trans-cleavage ofa stem-loop RNA substrate by a ribozyme derived from Neurospora VS RNA.EMBO J. (1995), 14(2), 368-76. ^(xxvi).Scott, W. G., Finch, J. T.,Aaron, K. The crystal structure of an all RNA hammerhead ribozyme: Aproposed mechanism for RNA catalytic cleavage. Cell, (1995), 81,991-1002. ^(xxvii).McKay, Structure and function of the hammerheadribozyme: an unfinished story. RNA, (1996), 2, 395-403. ^(xxviii).Long,D., Uhlenbeck, O., Hertel, K. Ligation with hammerhead ribozymes. U.S.Pat. No. 5,633,133. ^(xxix).Hertel, K. J., Herschlag, D., Uhlenbeck, O.A kinetic and thermodynamic framework for the hammerhead ribozymereaction. Biochemistry, (1994) 33, 3374-3385. Beigelman, L., et al.,Chemical modifications of hammerhead ribozymes. J. Biol. Chem., (1995)270, 25702-25708. ^(xxx).Beigelman, L., et al., Chemical modificationsof hammerhead ribozymes. J. Biol. Chem., (1995) 270, 25702-25708.^(xxxi).Hampel, Arnold; Tritz, Richard; Hicks, Margaret; Cruz, Phillip.‘Hairpin’ catalytic RNA model: evidence for helixes and sequencerequirement for substrate RNA. Nucleic Acids Res. 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Kinetics and Thermodynamicsof Intermolecular Catalysis by Hairpin Ribozymes. Biochemistry (1995),34(48), 15813-28. ^(xxxvii).Grasby, Jane A.; Mersmann, Karin; Singh,Mohinder; Gait, Michael J.. Purine Functional Groups in EssentialResidues of the Hairpin Ribozyme Required for Catalytic Cleavage of RNA.Biochemistry (1995), 34(12), 4068-76. ^(xxxviii).Schmidt, Sabine;Beigelman, Leonid; Karpeisky, Alexander; Usman, Nassim; Sorensen, UlrikS.; Gait, Michael J.. Base and sugar requirements for RNA cleavage ofessential nucleoside residues in internal loop B of the hairpinribozyme: implications for secondary structure. Nucleic Acids Res.(1996), 24(4), 573-81. ^(xxxix).Perrotta, Anne T.; Been, Michael D..Cleavage of oligoribonucleotides by a ribozyme derived from thehepatitis .delta. virus RNA sequence. Biochemistry (1992), 31(1), 16-21.^(xl).Perrotta, Anne T.; Been, Michael D.. 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TABLE II 2.5 μmol RNA Synthesis Cycle Wait Reagent Equivalents AmountTime* Phosphoramidites 6.5 163 μL 2.5 S-Ethyl Tetrazole 23.8 238 μL 2.5Acetic Anhydride 100 233 μL 5 sec N-Methyl Imidazole 186 233 μL 5 secTCA 83.2 1.73 mL  21 sec Iodine 8.0 1.18 mL  45 sec Acetonitrile NA 6.67mL  NA *Wait time does not include contact time during delivery.

TABLE IIIA Ribozyme K_(obs) (min⁻¹) K_(rel) A 4 1 B 0.7 0.2 C 0.0020.0005 D 0.004 0.001

TABLE IIIB Ribozyme K_(obs) (min⁻¹) K_(rel) A 0.32938 1.00 E 0.293950.89 F 0.22249 0.68 G 0.28201 0.86 A - all RNA 36mer Hammerhead Ribozyme

                   #             SEQUENCE LISTING<160> NUMBER OF SEQ ID NOS: 4 <210> SEQ ID NO 1 <211> LENGTH: 15<212> TYPE: RNA <213> ORGANISM: Artificial Sequence <220> FEATURE:<223> OTHER INFORMATION: Description of Artificial  #Sequence:  Nucleic      acid substrate <220> FEATURE: <221> NAME/KEY: misc_feature<222> LOCATION: (1)..(7) <223> OTHER INFORMATION: n stands for any nucle#otide <220> FEATURE: <221> NAME/KEY: misc_feature<222> LOCATION: (9)..(9) <223> OTHER INFORMATION: n stands for any nucle#otide <220> FEATURE: <221> NAME/KEY: misc_feature<222> LOCATION: (13)..(15)<223> OTHER INFORMATION: n stands for any nucle #otide <400> SEQUENCE: 1nnnnnnnyng hynnn               #                   #                  #    15 <210> SEQ ID NO 2 <211> LENGTH: 47 <212> TYPE: RNA<213> ORGANISM: Artificial Sequence <220> FEATURE:<223> OTHER INFORMATION: Description of Artificial #Sequence:  Enzymatic       Nucleic acid <220> FEATURE:<221> NAME/KEY: misc_feature <222> LOCATION: (1)..(4)<223> OTHER INFORMATION: n stands for any nucle #otide <220> FEATURE:<221> NAME/KEY: misc_feature <222> LOCATION: (9)..(19)<223> OTHER INFORMATION: n stands for any nucle #otide <220> FEATURE:<221> NAME/KEY: misc_feature <222> LOCATION: (25)..(31)<223> OTHER INFORMATION: n stands for any nucle #otide <220> FEATURE:<221> NAME/KEY: misc_feature <222> LOCATION: (39)..(47)<223> OTHER INFORMATION: n stands for any nucle #otide <400> SEQUENCE: 2nnnngaagnn nnnnnnnnna aahannnnnn nacauuacnn nnnnnnn   #                47 <210> SEQ ID NO 3 <211> LENGTH: 14 <212> TYPE: RNA<213> ORGANISM: Artificial Sequence <220> FEATURE:<223> OTHER INFORMATION: Description of Artificial  #Sequence:  Nucleic      acid substrate <400> SEQUENCE: 3 agggauuaug gaga              #                   #                   #     14 <210> SEQ ID NO 4<211> LENGTH: 36 <212> TYPE: RNA <213> ORGANISM: Artificial Sequence<220> FEATURE: <223> OTHER INFORMATION: Description of Artificial #Sequence:  Enzymatic       Nucleic Acid <220> FEATURE:<221> NAME/KEY: misc_feature <222> LOCATION: (1)..(8)<223> OTHER INFORMATION: 2′-O-Methyl <220> FEATURE:<221> NAME/KEY: misc_feature <222> LOCATION: (14)..(26)<223> OTHER INFORMATION: 2′-O-Methyl <220> FEATURE:<221> NAME/KEY: misc_feature <222> LOCATION: (28)..(29)<223> OTHER INFORMATION: 2′-O-Methyl <220> FEATURE:<221> NAME/KEY: misc_feature <222> LOCATION: (31)..(36)<223> OTHER INFORMATION: 2′-O-Methyl <220> FEATURE:<221> NAME/KEY: misc_feature <222> LOCATION: (9)..(9)<223> OTHER INFORMATION: 2′Amino <220> FEATURE:<221> NAME/KEY: misc_feature <222> LOCATION: (12)..(12)<223> OTHER INFORMATION: 2′Amino <220> FEATURE:<221> NAME/KEY: misc_feature <222> LOCATION: (27)..(27)<223> OTHER INFORMATION: Xyloguanosine <400> SEQUENCE: 4ucuccaucug augaggccga aaggccgaaa aucccu       #                  #       36

What is claimed is:
 1. A compound having the formula I:

wherein, R₁ is OH, O—R₃, wherein R₃ is independently a moiety selectedfrom the group consisting of alkyl, alkenyl, alkynyl, aryl, alkylaryl,carbocyclic aryl, heterocyclic aryl, amide and ester, C—R₃, wherein R₃is independently a moiety selected from the group consisting of alkyl,alkenyl, alkynyl, aryl, alkylaryl, carbocyclic aryl, heterocyclic aryl,amide and ester, halo, NHR₄, wherein R₄ is independently a moietyselected from the group consisting of alkyl (C1-22), acyl (C1-22),substituted or unsubstituted aryl, or OCH₂SCH₃ (methylthiomethyl),ONHR₅, wherein R₅ is independently a moiety selected from the groupconsisting of H, aminoacyl group, peptidyl group, biotinyl group,cholesteryl group, lipoic acid residue, retinoic acid residue, folicacid residue, ascorbic acid residue, nicotinic acid residue,6-aminopenicillanic acid residue, 7-aminocephalosporanic acid residue,alkyl, alkenyl, alkynyl, aryl, alkylaryl, carbocyclic aryl, heterocyclicaryl, amide or ester, ON=R₆, wherein R₆ is independently a moietyselected from the group consisting of pyridoxal residue,pyridoxal-5-phosphate residue, 13-cis-retinal residue, 9-cis-retinalresidue, alkyl, alkenyl, alkynyl, alkylaryl, carbocyclic alkylaryl, orheterocyclic alkylaryl; B is independently a nucleotide base orhydrogen; X is independently a phosphorus-containing group; and R₂ isindependently a blocking group or a phosphorus-containing group.
 2. Thecompound of claim 1, wherein said compound is a nucleotide.
 3. Thecompound of claim 1, wherein said compound is a nucleotide-triphosphate.4. A polynucleotide comprising the compound of claim 1 at one or morepositions.
 5. The polynucleotide of claim 4, wherein said polynucleotideis an enzymatic nucleic acid.
 6. The enzymatic nucleic acid of claim 5,wherein said nucleic acid is in a hammerhead configuration.
 7. Theenzymatic nucleic acid of claim 6, wherein said nucleic acid is in ahairpin configuration.
 8. The enzymatic nucleic acid of claim 6, whereinsaid nucleic acid is in a hepatitis delta virus, group I intron, VS RNA,group II intron or RNase P RNA configuration.
 9. The compound of claim1, wherein said compound is xylo riboadenosine.
 10. The compound ofclaim 1, wherein said compound is xylo riboguanosine.
 11. The compoundof claim 1, wherein said compound is xylo ribonucleosidephosphoramidite.
 12. The compound of claim 11, wherein said compound isxylo riboguanosine phosphoramidite.
 13. The compound of claim 11,wherein said compound is xylo riboadenosine phosphoramidite.
 14. Apharmaceutical composition comprising a compound of claim
 1. 15. Apharmaceutical composition comprising a polynucleotide of claim
 5. 16.The compound of claim 1, wherein said compound is used as an antiviralagent.